Implantable cardiac rhythm management (CRM) systems can be used to help patients susceptible to dangerous high-rate cardiac events. Conventional CRM systems utilize electrodes which are placed in or on the heart for sensing and to pace and shock the heart. The most common such system is a transvenous system, which uses a lead that is threaded through the vasculature and into the heart for attachment in a heart chamber or in a blood vessel of the heart itself. Before the advent of transvenous technology, systems were placed epicardially, with electrodes placed on exterior of the heart in an open-chest surgery. A recent development in the world of CRM systems is the introduction of subcutaneous-only implantable systems, including the S-ICD® system, a subcutaneous implantable defibrillator manufactured by Cameron Health, Inc.
Therapy in CRM systems is often directed at potentially deadly high rate cardiac events. High rate cardiac events may include atrial fibrillation, atrial tachycardia, ventricular fibrillation and/or ventricular tachycardia, among others. Atrial high rate events are often less dangerous and, though possible to treat with implantable systems, are more often pharmacologically mediated. Ventricular high rate events can be more dangerous and are often the target of therapy with implantable devices. Currently two types of therapy for high rate ventricular events are defibrillation and pacing.
Defibrillation is the delivery of high amplitude pulses to the heart, often measured in joules, to correct disorganized rhythms that impair the ability of one or more chambers of the heart to pump blood. The purpose of defibrillation is to depolarize a sufficient mass of the myocardium to allow normal cardiac signals and cycles to resume. If high amplitude therapy is delivered synchronous with a cardiac “beat” in the presence of an atrial tachyarrhythmia, it may be referred to as cardioversion. High energy shocks typically use up to 1000 volts, or more, with durations of up to 30 milliseconds. Total energy for transvenous systems is often in the range of up to 40 Joules, and subcutaneous systems may use up to 80 Joules or more.
A series of high rate pacing stimuli can also be used to treat some tachyarrhythmias. This series of high rate pacing pulses is called Antitachycardia Pacing (ATP). ATP is delivered as a series of pacing pulses by a transvenous or epicardial electrode and can be delivered by an implanted transvenous system in a manner that is not perceptible to the patient, in contrast to defibrillation, which patients notice unless the arrhythmia has rendered them unconscious. In contrast to disorganized arrhythmias that require a defibrillation shock, organized or monomorphic arrhythmias may sometimes be effectively terminated using ATP. Transvenous pacing pulses for ATP typically use up to approximately 7.5 Volts and may have durations up to about 1.5 milliseconds, such that energy levels are well under a single joule. Both pacing and defibrillation can be delivered as monophasic or biphasic waveforms; multiphasic waveforms are known and have been studied but are not particularly common in the field. Subcutaneous systems may use higher energies, for example in the range of up to 40 volts or more; one specific example uses a 200 milliamp constant current biphasic pace pulse with 7.5 milliseconds duration in each phase. While prior discussions of subcutaneous CRM systems have noted the potential applicability of both defibrillation therapy and ATP in subcutaneous CRM systems (see, for example, U.S. Pat. Nos. 6,952,610 and/or 6,856,835) additional or further alternatives are sought.